PROJECT SUMMARY Despite technological advances, arterial revascularization ? especially peripheral angioplasty and bypass ? often fail due to intimal hyperplasia (IH). The primary pathology of IH is the transformation of vascular smooth muscle cells (SMCs) from a quiescent state to an activated state, with multiple IH-promoting phenotypes. Currently, prevention of IH focuses on anti-proliferative agents with limited success. Moreover, a lack of insight exists into master proximal regulators that govern all of the major SMC pathogenic phenotypes, not only proliferation/migration, but also de-differentiation and inflammation. Identification and effective targeting of such master regulators would represent a major advance in anti-IH therapy. Recently, our team has identified that the bromo and extraterminal (BET) family of proteins, termed epigenetic readers, may uniquely fit this master role. Recent groundbreaking studies have revealed that the BET family of proteins can change the phenotype of different cell types, by coupling two acetyl-binding bromo-domains with acetylated key transcription factors consequently activating the transcription of a select subset of genes. We have made the exciting observation that the first-in-class BET inhibitor (JQ1) halts SMC pathogenic transformation and mitigates IH. This leads to our central hypothesis, that unlike downstream individual pathways that are subject to redundancy, BET proteins act as upstream drivers of nodal transcription factors (such as STAT3) which in turn determine downstream pathogenic SMC phenotypes. Since JQ1 globally blocks both bromo-domains in all three BET proteins, in this proposal we will differentiate the functions of the various BET proteins and bromo-domains in SMC transformation. In Aim1 we will determine which of the BET proteins dictate SMC pathogenic phenotypes and IH. We have observed that two BET proteins dramatically increase after arterial injury. In Aim2 we will further delineate which of the two bromo-domains is responsible for the BET pathogenic function. The rationale is based on our data that blocking the two bromo-domains (each in all three BET proteins) with respective inhibitors produces differential effects on SMC proliferation and inflammation. In Aim3 we will identify the key transcription factors that are governed by BET proteins, thereby mediating the BET function on SMC transformation. We have preliminarily identified STAT3 as a leading candidate. Our ultimate goal is to elucidate the specific BET protein or bromo-domain that is the most opportune target in the prevention of intimal hyperplasia. To this end, we will make use of the revolutionary CRISPR/Cas9 technology for BET protein knockout and bromo-domain mutation in the mouse genome. The proposal is novel because we will identify a key epigenetic switch that controls SMC pathogenic transformation. The research is significant because we will establish a BET protein or bromo-domain as a novel target for developing optimized therapeutic methods to treat IH and recurrent vascular disease. 1